1. Field of the Invention
This invention relates to a circuit for direct current compensation and detection for transformer coupled circuits.
2. Description of the Prior Art
In the past, two problems often encountered in telephone systems have required independent solutions. One of these problems pertains to providing direct current compensation of transformers used to separate a subscriber line and a telephone switching system. The other problem pertains to detecting subscriber signaling.
Transformers are typically employed in telephone systems to provide direct current separation between subscriber loops and switching networks. One problem that has been encountered is that a direct current flow in the subscriber loop may cause saturation of the transformer thereby causing distortion of alternating current signals transmitted through the transformer. A direct current compensation circuit solving this problem is described in U.S. Pat. No. 3,714,548 which issued Jan. 30, 1973 to M.S. Macrander and which is assigned to the assignee of this invention. That compensation circuit is connected to a subscriber loop and to a compensation winding of the transformer and is arranged so that any direct current flow in the subscriber loop will result in a corresponding direct current flow in the compensation winding so that magnetic fields produced in the transformer by the direct currents cancel each other.
Direct current signaling is typically employed in subscriber loops to indicate to telephone switching equipment whether a subscriber line is idle or busy, to initiate service requests and to transmit called number information in the form of dial pulses. With such signaling each subscriber loop is opened to indicate an idle condition, closed to indicate a busy condition or a request for service, or closed and opened in sequence to indicate dial pulses. Each time the loop is closed, direct current from the switching equipment flows in the loop.
Telephone switching equipment has included various circuits to detect signaling information. Typical of these past approaches to signaling detection are the detector circuits shown in U.S. Pat. No. 3,579,106 which issued May 18, 1971 to M.R. Lord and in U.S. Pat. No. 3,781,571 which issued Dec. 25, 1973 to M.R. Lord. In U.S. Pat. No. 3,579,106 a loop current detector is described wherein the currents flowing in both legs of a loop are amplified independently and thereafter combined. The combined current is compared against a reference current to produce an output signal when the combined current exceeds the reference current by a predetermined amount sufficient to bias a threshold network. In the detector circuit described in the above-referenced U.S. Pat. No. 3,781,571 a resistor is connected in each leg of the loop. A direct current amplifier coupled to one of the resistors provides an output signal proportional to the loop current. The output of the amplifier is coupled to the input of a voltage threshold detector. The voltage drop across the other resistor is also coupled to the input of the threshold detector. The threshold detector provides an output to indicate when the input voltage is less or greater than a reference voltage.
Other past approaches to signaling detection employed bridge-type signaling detectors such as shown in U.S. Pat. No. 3,821,486 which issued June 28, 1974 to H.W. Mussman and in U.S. Pat. No. 3,156,778 which issued Nov. 10, 1964 to F.P. Cirone or non-linear resistance elements in the subscriber loop such as shown in Great Britain Pat. No. 1,000,362 to D.C. Emmonds, published Aug. 4, 1965.